A variety of studies are contemplated to elucidate the physiological and biochemical characteristics of specific triiodothyronine nuclear binding sites which have recently been reported by this laboratory. The kinetics of the interchange between T3 and these sites will be investigated. A large number of thyroid hormone analogues will be tested for their ability to bind to these sites. Nuclear binding in a number of tissues and in a variety of physiological states will be assessed to determine the relationship between nuclear binding and hormonal activity.